There’s only a short window—perhaps 3-4 years—between which a child (1) can comprehend, and (2) still believes, in Santa.
My nephew is firmly in that window, and everyone in our family is relishing watching his joy. He skipped and laughed his way to bed (making sure he was asleep before Santa arrived), but not before he put out milk, cookies, and a little something for the reindeer.
It’s a window that comes and goes quickly, so we’re savouring it.